Aggregation and fibrillation of SODl have been implicated in disease mechanisms of Amyotrophic Lateral Sclerosis (ALS), and it is a major new goal of this Program Project renewal to develop better biological assays to study the toxicity of these multimeric forms of SODl in systems that will be more relevant to the disease in humans. In Project 2 we further develop and use a human cell culture system that closely models important cell biological aspects of motor neuron degeneration[unreadable]our recently developed human embryonic stem cell-derived motor neuron (HESC-MN) system. The cells have distinct advantages over other model systems as they represent the major cell type that degenerates in ALS and they are fully human. The cells express identifying neuronal markers, exhibit electrophysiological function typical for mature motor neurons, and can be co-cultured with other neuronal and non-neuronal cells. Transfection of these cells to express ALS-SODl proteins causes deleterious effeds on cell survival and morphology. Importantly for this project, we have recently shown that exogenously added ALS-SODl protein multimers are taken up quite well. We will utilize these cells to study the toxicity of SODl protein multimers and aggregates at different stages of their formation and relate it to the progression of motor neuron degeneration. This research plan outlines a highly collaborative, step-by-step approach to evaluate spontaneous and induced mutant and WT SODl aggregate formation in motor neurons, followed by an investigation of the consequences of SODl aggregates on neurodegenerative mechanisms and, finally, by using pharmacological inhibitors of SODl aggregation to investigate whether reduced SODl aggregation can prevent motor neuron death.